Vapour compression systems, such as refrigeration systems, air condition systems or heat pumps, normally comprise a heat rejecting heat exchanger arranged to exchange heat with a secondary fluid flow across the heat rejecting heat exchanger in such a manner that heat is rejected from the vapour compression system and transferred to the secondary fluid flow. The heat rejecting heat exchanger may, e.g., be in the form of a condenser or in the form of a gas cooler.
Previously, an outlet temperature of refrigerant leaving the heat rejecting heat exchanger was expected to decrease slowly as a function of an increase in fan speed of the one or more fans arranged to cause the secondary fluid flow across the heat rejecting heat exchanger. Some vapour compression systems have been provided with heat recovery systems arranged to recover heat from the refrigerant immediately before the refrigerant reaches the heat rejecting heat exchanger, and use the recovered heat in other parts of the vapour compression system or in systems for external to the vapour compression system.
The heat recovery has the consequence that the refrigerant which reaches the heat rejecting heat exchanger has already been cooled, and thereby the heat needing to be rejected from the vapour compression system by the heat rejecting heat exchanger is considerably reduced. As a consequence, the heat rejecting heat exchanger may be over-dimensioned. The outlet temperature of refrigerant leaving the heat rejecting heat exchanger decreases drastically as the speed of the fan increases. This makes it difficult to control the fan or fans, because small variations in fan speed cause significant variations in the outlet temperature of refrigerant leaving the heat rejecting heat exchanger, thereby causing instability. Furthermore, at low fan speed, the response in outlet temperature to changes in the fan speed is very strong, while at high fan speed, the response in outlet temperature to changes in the fan speed is very weak. The optimal operating point for the fan speed is exactly at the point where the response in outlet temperature changes from strong to weak. This makes it even more difficult to control the fan speed dynamically. This problem has previously been solved by simply allowing the fan or fans to operate continuously at a high rotational speed, e.g. at or close to maximum rotational speed. However, this causes a relatively high electrical energy consumption of the fan or fans.
U.S. Pat. No. 5,086,626 discloses an air conditioner with function for temperature control of radiant heat exchanger. A fan delivers air to an indoor heat exchanger. A temperature sensor detects the temperature of the radiant heat exchanger. A controller controls the indoor heat exchanger fan speed for controlling the radiant heat temperature from the radiant heat exchanger in accordance with a temperature detection signal from the sensor. In U.S. Pat. No. 5,086,626 the fan speed is controlled on the basis of a measured surface temperature of the radiant heat exchanger, and not on the basis of a temperature of refrigerant leaving the radiant heat exchanger.